1,3-difluoronapthalene derivatives for liquid crystal mixtures

ABSTRACT

1,3-Difluoronaphthalene derivatives of the formula (I) 
     
         R.sup.1 (--A.sup.1 --M.sup.1).sub.a (--A.sup.2 --M.sup.2).sub.b 
    
      --B(--M 3  --A 3 ) c  (--M 4  --A 4 ) d  --R 2  (I) 
     in which 
     B is ##STR1## and R 1  (--A 1  --M 1 ) a  (--A 2  --M 2 ) and (--M 3  --A 3 )(--M 4  --A 4 )R 2  denote mesogenic radicals are suitable as components for liquid-crystal mixtures, especially ferroelectric mixtures.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a 371 of PCT/EP97/06997 filed Dec. 12, 1997.

This application claims the priority benefits of German PatentApplication No. 196 52 252.8, filed on Dec. 16, 1996.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the synthesis of 1,3-difluoronaphthalenederivatives for making ferroeletric liquid crystal mixtures and theiruses in ferroelectric switching and display device.

2. Description of the Related Art

In addition to nematic and cholesteric liquid crystals, recent timeshave also seen the use of optically active tilted smectic(ferroelectric) liquid crystals in commercial display devices.

Clark and Lagerwall showed that the use of ferroelectric liquid crystals(FLCs) in very thin cells leads to optoelectric switching or displayelements having response times faster by a factor of up to 1000 thanthose of the conventional TN (twisted nematic) cells (see, for example,EP-A 0 032 362). On the basis of these and other favorable properties,for example the possibility of bistable switching and the contrast,which is virtually independent of viewing angle, FLCs are in principlehighly suited to applications such as computer displays.

For the use of FLCs in electrooptical or completely optical assembliesthere is a need either for compounds which form tilted or orthogonalsmectic phases and which are themselves optically active, or else forcompounds which, although forming such smectic phases are not themselvesoptically active, can be doped with optically active compounds to induceferroelectric smectic phases. The desired phase should at the same timebe stable over as wide as possible a temperature range.

Obtaining a good contrast ratio in electrooptical assembliesnecessitates a uniform planar orientation of the liquid crystals. Goodorientation in the S_(A) and S*_(C) phase can be achieved, for example,when the phase sequence of the liquid-crystal mixture with decreasingtemperature is as follows:

    isotropic→N*→S.sub.A →S*.sub.C

A precondition is that the pitch of the helix in the N* phase is verylarge (greater than 10 μm) or, even better, is fully compensated (see,for example, T. Matsumoto et al., Proc. of the 6th Int. Display ResearchConf., Japan Display, Sep. 30-Oct. 2, 1986, Tokyo, Japan, pp. 468-470;M. Murakami et al., ibid. pp. 344-pp. 347). This is done, for example,by adding one or more optically active dopes which induce, say, aright-handed helix to the chiral liquid-crystal mixture which in the N*phase has a left-handed helix, in amounts such that the helix iscompensated.

For the use of the SSFLCD effect (Surface Stabilized FerroelectricLiquid Crystal Display) of Clark and Lagerwall for uniform planarorientation a further precondition is that the pitch in the smectic C*phase is substantially greater than the thickness of the display element(Mol. Cryst. Liq. Cryst. 1983, 94, 213 and 1984, 114, 151).

The optical response time T [μs] of ferroelectric liquid-crystalsystems, which should be as short as possible, depends on the rotationalviscosity of the system γ [mPas], on the spontaneous polarization P_(S)[nC/cm² ] and on the electric field strength E [V/m] in accordance withthe relationship ##EQU1##

Since the field strength E is determined by the electrode separation inthe electrooptical component and by the applied voltage, theferroelectric display medium must be of low viscosity and must have ahigh spontaneous polarization in order for a short response time to beobtained.

Finally, requirements in addition to thermal, chemical and photochemicalstability are for a small optical anisotropy Δn and a small positive or,preferably, negative dielectric anisotropy Δε (see, for example, S. T.Lagerwall et al., "Ferroelectric Liquid Crystals for Displays", SIDSymposium, Oct. Meeting 1985, San Diego, Calif., U.S.A.).

The entirety of these requirements can only be met with mixtures of twoor more components. The basis of these mixtures (or matrix) ispreferably formed by compounds which as far as possible themselvesalready have the desired phase sequence I→N→S_(A) →S_(C). Furthercomponents are often added to the mixture in order to lower the meltingpoint and to broaden the S_(C) phase and usually the N phase as well,for inducing the optical activity, for pitch compensation and foradapting the optical and dielectric anisotropy, as far as possiblewithout increasing the rotational viscosity, for example.

Ferroelectric liquid-crystal displays can also be operated by utilizingthe DHF (Distorted Helix Formation) effect or the PSFLCD (PitchStabilized Ferroelectric Liquid Crystal Display, also called SBF=ShortPitch Bistable Ferroelectric) effect. The DHF effect was described by B.I. Ostrovski in Advances in Liquid Crystal Research and Applications,Oxford/Budapest, 1980, 469 ff. and the PSFLCD effect is described inDE-A 39 20 625 and EP-A 0 405 346. In contrast to the SSFLCD effect,utilizing these effects requires a liquid-crystalline material with ashort S_(C) pitch.

Naphthalene derivatives for use in liquid-crystal mixtures are known,for example, from WO-A 92/16 500. 1,2-Difluoronaphthalene derivativesare known from DE-A 195 22 167.

However, since the development--of ferroelectric liquid-crystal mixturesin particular--can in no way be regarded as complete, the manufacturersof displays are interested in a very wide variety of components formixtures. One of the reasons for this is that only the interaction ofthe liquid-crystalline mixtures with the individual components of thedisplay device or of the cells (for example the alignment layer) allowsconclusions to be drawn about the quality of the liquid-crystallinemixtures too.

The object of the present invention was therefore to provide novelcompounds which are suitable in liquid-crystalline mixtures forimproving the profile of properties of these mixtures.

BRIEF SUMMARY OF THE INVENTION

It has now been found, surprisingly, that 1,3-difluoronaphthalenederivatives of the formula (I) are particularly suitable for use inliquid-crystal mixtures.

DESCRIPTION OF THE INVENTION

The invention therefore provides 1,3-difluoronaphthalene derivatives ofthe formula (I)

    R.sup.1 (--A.sup.1 --M.sup.1).sub.a (--A.sup.2 --M.sup.2).sub.b --B(--M.sup.3 --A.sup.3).sub.c (--M.sup.4 --A.sup.4).sub.d --R.sup.2 (I)

in which the symbols and indices have the following meanings:

group B is ##STR2## R¹ and R² are identical or different and are a)hydrogen, --F, --Cl, --CF₃, --OCF₃ or --CN,

b) a straight-chain or branched alkyl radical (with or without anasymmetric carbon atom) having 1 to 20 carbon atoms, where

b1) one or more nonadjacent and nonterminal CH₂ groups can be replacedby --O--, --S--, --CO--O--, --O--CO--, --O--CO--O-- or --Si(CH₃)₂ --,and/or

b2) one or more CH₂ groups can be replaced by --CH═CH--, --C.tbd.C--,cyclopropane-1,2-diyl, 1,4-phenylene, 1,4-cyclohexylene or1,3-cyclopentylene, and/or

b3) one or more H atoms can be replaced by F and/or Cl, and/or

b4) the terminal CH₃ group can be replaced by one of the followingchiral groups (optically active or racemic): ##STR3## with the provisothat at most one of the radicals R¹, R² is hydrogen, --F, --Cl, --CF₃,--OCF₃ or --CN;

R³, R⁴, R⁵, R⁶ and R⁷ are identical or different and are

a) hydrogen,

b) a straight-chain or branched alkyl radical (with or without anasymmetric carbon atom) having 1 to 16 carbon atoms, where

b1) one or more nonadjacent and nonterminal CH₂ groups can be replacedby --O--, and/or

b2) one or two CH₂ groups can be replaced by --CH═CH--,

c) R⁴ and R⁵ together are alternatively --(CH₂)₄ -- or --(CH₂)₅ --, ifthey are attached to an oxirane, dioxolane, tetrahydrofuran,tetrahydropyran, butyrolactone or valerolactone system;

M¹, M², M³ and M⁴ are identical or different and are --CO--O--,--O--CO--, --CO--S--, --S--CO--, --CS--O--, --O--CS--, --CH₂ --O--,--O--CH₂ --, --CH₂ --S--, --S--CH₂ --, --CH═CH--, --C.tbd.C--, --CH₂--CH₂ --CO--O--, --O--CO--CH₂ --CH₂ -- or a single bond;

A¹, A², A³ and A⁴ are identical or different and are 1,4-phenylene inwhich one or more H atoms can be replaced by F, Cl and/or CN,pyrazine-2,5-diyl, in which one or two H atoms can be replaced by F, Cland/or CN, pyridazine-3,6-diyl in which one or two H atoms can bereplaced by F, Cl and/or CN, pyridine-2,5-diyl in which one or more Hatoms can be replaced by F, Cl and/or CN, pyrimidine-2,5-diyl in whichone or two H atoms can be replaced by F, Cl and/or CN, 1,4-cyclohexylenein which one or two H atoms can be replaced by CN and/or CH₃ and/or F,1,3,4-thiadiazole-2,5-diyl, 1,3-dioxane-2,5-diyl, 1,3-dithiane-2,5-diyl,1,3-thiazole-2,4-diyl, in which one H atom can be replaced by F, Cland/or CN, 1,3-thiazole-2,5-diyl in which one H atom can be replaced byF, Cl and/or CN, thiophene-2,4-diyl, in which one H atom can be replacedby F, Cl and/or CN, thiophene-2,5-diyl in which one or two H atoms canbe replaced by F, Cl and/or CN, naphthalene-2,6-diyl in which one ormore H atoms can be replaced by F, Cl and/or CN, or 1-(C₁-C₄)alkyl-1-silacyclohexylene-1,4-diyl;

a, b, c and d are 0 or 1, with the proviso that the compound of theformula (I) contains no more than four ring systems having five or moremembers.

The provision of compounds of the formula (I) considerably broadens, ingeneral terms, the palette of liquid-crystalline substances which from avariety of applications-related standpoints are suitable for thepreparation of liquid-crystalline mixtures.

In this connection the compounds of the formula (I) possess a broadscope of application. Depending on the selection of the substituentsthey can be used as base materials forming the predominant part ofliquid-crystalline phase compositions; or alternatively, compounds ofthe formula (I) can also be added to liquid-crystalline base materialsfrom other classes of compounds, in order for example to influence thedielectric and/or optical anisotropy of such a dielectric materialand/or to optimize its threshold voltage and/or its viscosity.

The compounds of the formula (I) are particularly suitable for additioneven in small amounts for influencing the dielectric anisotropy (Δε)toward higher negative values.

The novel compounds of the formula (I) are particularly suitable for usein FLC mixtures for ferroelectric switching and/or display devices,which are operated in inverse mode.

The symbols and indices in the formula (I) preferably have the followingmeanings:

R¹ and R² are, preferably, identical or different and are

a) hydrogen,

b) a straight-chain or branched alkyl radical (with or without anasymmetric carbon atom) having 1 to 18 carbon atoms, where

b1) one or more nonadjacent and nonterminal CH₂ groups can be replacedby --O--, --CO--O--, --O--CO--, --O--CO--O or --Si(CH₃)₂ --, and/or

b2) one CH₂ group can be replaced by cyclopropane-1,2-diyl,1,4-phenylene or trans-1,4-cyclohexylene, and/or

b3) one or more H atoms can be replaced by F, and/or

b4) the terminal CH₃ group can be replaced by one of the followingchiral groups (optically active or racemic): ##STR4## with the provisothat at most one of the radicals R¹ and R² is hydrogen.

R¹ and R² are, with particular preference, identical or different andare

a) hydrogen,

b) a straight-chain or branched alkyl radical (with or without anasymmetric carbon atom) having 1 to 16 carbon atoms, where

b1) one or two nonadjacent and nonterminal CH₂ groups can be replaced by--O--, --CO--O--, --O--CO--, --O--CO--O-- or --Si(CH₃)₂ --, and/or

b2) one CH₂ group can be replaced by 1,4-phenylene ortrans-1,4-cyclohexylene, and/or

b3) one or more H atoms can be replaced by F, and/or

b4) the terminal CH₃ group can be replaced by one of the followingchiral groups (optically active or racemic): ##STR5## with the provisothat at most one of the radicals R¹ and R² is hydrogen.

R³, R⁴, R⁵, R⁶ and R⁷ are, preferably, identical or different and are

a) hydrogen,

b) a straight-chain or branched alkyl radical (with or without anasymmetric carbon atom) having 1 to 14 carbon atoms, where

b1) one or two nonadjacent and nonterminal CH₂ groups can be replaced by--O--, and/or

b2) one CH₂ group can be replaced by --CH═CH--,

c) R⁴ and R⁵ together are alternatively --(CH₂)₄ -- or --(CH₂)₅ --, ifthey are attached to an oxirane, dioxolane, tetrahydrofuran,tetrahydropyran, butyrolactone or valerolactone system.

R³, R⁴, R⁵, R⁶ and R⁷ are, with particular preference, identical ordifferent and are

a) hydrogen,

b) a straight-chain or branched alkyl radical (with or without anasymmetric carbon atom) having 1 to 14 carbon atoms, where

b1) one nonterminal CH₂ group can be replaced by --O--, and/or

c) R⁴ and R⁵ together are alternatively --(CH₂)₄ -- or --(CH₂)₅ --, ifthey are attached to an oxirane, dioxolane, tetrahydrofuran,tetrahydropyran, butyrolactone or valerolactone system.

M¹, M², M³ and M⁴ are, preferably, identical or different and are--CO--O--, --O--CO--, --CH₂ --O--, --O--CH₂ --, --CH═CH--, --C.tbd.C--,--CH₂ --CH₂ --CO--O, --O--CO--CH₂ --CH₂ -- or a single bond.

M¹, M², M³ and M⁴ are, with particular preference, identical ordifferent and are --CO--O--, --O--CO--, --CH₂ --O--, --O--CH₂ -- or asingle bond.

A¹, A², A³ and A⁴ are, preferably, identical or different and are1,4-phenylene in which one or two H atoms can be replaced by F and/orCN, pyridine-2,5-diyl, in which one or two H atoms can be replaced by Fand/or CN, pyrimidine-2,5-diyl in which one or two H atoms can bereplaced by F, trans-1,4-cyclohexylene in which one or two H atoms canbe replaced by CN and/or CH₃ and/or F, 1,3,4-thiadiazole-2,5-diyl,1,3-dioxane-2,5-diyl, 1,3-thiazole-2,4-diyl, in which one H atom can bereplaced by F and/or CN, 1,3-thiazole-2,5-diyl in which one H atom canbe replaced by F and/or CN, or thiophene-2,5-diyl in which one or two Hatoms can be replaced by F and/or CN.

A¹, A², A³ and A⁴ are, with particular preference, identical ordifferent and are 1,4-phenylene in which one or two H atoms can bereplaced by F, pyridine-2,5-diyl in which one H atom can be replaced byF, pyrimidine-2,5-diyl, trans-1,4-cyclohexylene in which one or two Hatoms can be replaced by CN and/or CH₃ and/or F.

Very particular preference is given to the following compounds of theformula (Ia) to (Ii): ##STR6## in which R¹ and R² have the meaningsstated above.

The compounds according to the invention are prepared by methods knownper se from the literature, as are described in standard works onorganic synthesis, for example Houben-Weyl, Methoden der OrganischenChemie, Georg-Thieme-Verlag, Stuttgart.

The preparation is carried out under reaction conditions which are knownand suitable for said reactions. Use may also be made here of variantswhich are known per se but which are not mentioned here in any moredetail.

If desired, the starting materials can also be formed in situ, by notisolating them from the reaction mixture but instead immediatelyconverting them further into the compounds of the formula (I).

By way of example, a synthesis route for the synthesis of1,3-difluoronaphthalene and to compounds of the formula (I) is indicatedin scheme 1, although other methods are also conceivable and possible.##STR7##

In Scheme 2, the synthesis of compounds of the formula I with an arylsubstitution in position 2 of the naphthalene system is stated by way ofexample. ##STR8##

The group R^(y) is equivalent to the group R¹ (--A¹ --M¹)_(a) (--A²--M²)_(b) -- or an appropriate unprotected or protected precursorthereof which in later steps can be converted into this group by methodswhich are known per se and are familiar to the skilled worker. The groupR^(x) is equivalent to the group (--M³ --A³)_(c) (--M⁴ --A⁴)_(d) --R² oran appropriate unprotected or protected precursor thereof which in latersteps can be converted into this group by methods which are known per seand are familiar to the skilled worker. Its preparation takes placeunder reaction conditions which are known and suitable for saidreactions. In this context it is also possible to make use of variantswhich are known per se and are not mentioned in any more detail here.

Reference may be made, for example, to DE-A 23 44 732, 24 50 088, 24 29093, 25 02 94, 26 36 684, 27 01 591 and 27 52 975 for compounds with1,4-cyclohexylene and 1,4-phenylene groups; to DE-A 26 41 724 forcompounds with pyrimidine-2,5-diyl groups; to DE-A 40 26 223 and EP-A 0391 203 for compounds with pyridine-2,5-diyl groups; to DE-A 32 31 462for compounds with pyridazine-3,6-diyl groups; and to EP-A 309 514 forcompounds with 1,3,4-thiadiazole-2,5-diyl groups; WO-A 92/16500 fornaphthalene-2,6-diyl groups; and EP-A 0 630 903 for compounds having1-sila-1,4-cyclohexylene groups.

The preparation of disubstituted pyridines, disubstituted pyrazines,disubstituted pyrimidines and disubstituted pyradazines is also given,for example, in the corresponding volumes of the series "The Chemistryof Heterocyclic Compounds" by A. Weissberger and E. C. Taylor (editors).

Dioxane derivatives are judiciously prepared by reaction of acorresponding aldehyde (or a reactive derivative thereof) with acorresponding 1,3-diol (or a reactive derivative thereof), preferably inthe presence of an inert solvent, such as benzene or toluene, and/or inthe presence of a catalyst, for example strong acid such as sulfuricacid, benzene- or p-toluenesulfonic acid, at temperatures between about20° C. and about 150° C., preferably between 80° C. and 120° C.Primarily suitable as reactive derivatives of the starting materials areacetals.

Some of said aldehydes and 1,3-diols and their reactive derivatives areknown while some can be prepared without difficulty by standard methodsof organic chemistry from compounds known from the literature. Forexample, the aldehydes are obtainable by oxidation of correspondingalcohols or by reduction of nitrites or corresponding carboxylic acidsor derivatives thereof, and the diols by reduction of correspondingdiesters.

Compounds in which an aromatic ring is substituted by at least one Fatom can also be obtained from the corresponding diazonium salts byreplacement of the diazonium group with a fluorine atom, for example bythe methods of Balz and Schiemann.

As far as the linking of ring systems to one another is concerned,reference may be made, for example, to:

N. Miyaura, T. Yanagai and A. Suzuki in Synthetic Communications 11(1981), 513-519, DE-C 39 30 663, M. J. Sharp, W. Cheng, V. Snieckus inTetrahedron Letters 28 (1987) 5093; G. W. Gray in J. Chem. Soc. PerkinTrans II 1989, 2041 and Mol. Cryst. Liq. Cryst. 172 (1989) 165, 204(1991) 43 and 91; EP-A 0 449 015; WO-A 89/12039; WO-A 89/03821; EP-A 0354 434 and EP-A 0 694 530 for the direct linking of aromatics andheteroaromatics; DE-A 32 01 721 for compounds with --CH₂ CH₂ -- bridges,and Koji Seto et al. in Liquid Crystals 8 (1990) 861-870 for compoundscontaining --C.tbd.C-- bridges.

Esters of the formula (I) can also be obtained by esterification ofcorresponding carboxylic acids (or reactive derivatives thereof) usingalcohols or phenols (or reactive derivatives thereof), in accordancewith the DCC method (DCC=dicyclohexylcarbodiimide) or analogously toDE-A 44 27 198.

The corresponding carboxylic acids and alcohols and phenols are known orcan be prepared analogously to known processes.

Particularly suitable reactive derivatives of said carboxylic acids arethe acid halides, especially the chlorides and bromides, and also theanhydrides, including mixed anhydrides, for example, azides or esters,especially alkyl esters having 1-4 carbon atoms in the alkyl group.

Particularly suitable reactive derivatives of said alcohols and phenolsare the corresponding metal alcoholates or phenolates, preferably of analkali metal such as sodium or potassium.

The esterification is advantageously carried out in the presence of aninert solvent. Particularly suitable solvents are ethers, such asdiethyl ether, di-n-butyl ether, THF, dioxane or anisole, ketones, suchas acetone, butanone or cyclohexanone, amides, such as DMF orhexamethylphosphoramide, hydrocarbons, such as benzene, toluene orxylene, halogenated hydrocarbons, such as tetrachloromethane,dichloromethane or tetrachloroethylene, and sulfoxides, such as dimethylsulfoxide or sulfolane.

Ethers of the formula (I) are obtainable by etherification ofcorresponding hydroxy compounds, preferably corresponding phenols, wherethe hydroxy compound is judiciously first of all converted into acorresponding metal derivative, for example into the correspondingalkali metal alcoholate or alkali metal phenolate by treatment with NaH,NaNH₂, NaOH, KOH, Na₂ CO₃ or K₂ CO₃. This product can then be reactedwith the corresponding alkyl halide, alkylsulfonate or dialkyl sulfate,judiciously in an inert solvent such as acetone, 1,2-dimethoxyethane,DMF or dimethyl sulfoxide, or else with an excess of aqueous oraqueous-alcoholic NaOH or KOH at temperatures of between about 20° and100° C.

Regarding the synthesis of specific radicals R¹ and R², reference mayadditionally be made, for example, to

EP-B 0 355 008 for compounds with silicon-containing side chains,

EP-B 0 292 954 for optically active compounds with an oxirane esterunit,

EP-B 0 263 437 for optically active compounds with an oxirane etherunit,

EP-B 0 361 272 for optically active compounds with a dioxolane esterunit,

EP-B 0 351 746 for optically active compounds with a dioxolane etherunit,

U.S. Pat. No. 5,051,506 for optically active compounds with a2,3-difluoroalkyloxy unit,

U.S. Pat. No. 4,798,680 for optically active compounds with a2-fluoroalkyloxy unit,

U.S. Pat. No. 4,855,429 for optically active compounds withα-chlorocarboxylic acid unit,

EP-A 0 552 658 for compounds with cyclohexylpropionic acid radicals, and

EP-A 0 318 423 for compounds with cyclopropyl groups in the side chain.

The compounds according to formula (I) are especially useful foroperating in smectic and nematic liquid crystal mixtures, in the case ofnematic mixtures preferably for active matrix displays (AM-LCD) (e.g. C.Prince, Seminar Lecture Notes, Volume I, p. M-3/3-M-22, SIDInternational Symposium 1997, B. B. Bahadur, Liquid Crystal Applicationsand Uses, Vol 1, p. 410, World Scientific Publishing, 1990, E. Luder,Recent Progress of AM LCD's, Proceedings of the 15^(th) InternationalDisplays Research Conference, 1995, p. 9-12) and "in-plane-switchingdisplays" (IPS-LCD), in the case of smectic liquid crystal mixturespreferably for chirally inclined smectic (ferroelectric orantiferroelectric) displays, for ECB displays (electrically controlledbirefringence) and for electroclinic displays.

The invention also provides for the use of compounds of the formula (I)in liquid-crystal mixtures, preferably smectic and nematic mixtures,especially ferroelectric mixtures.

Especially preferred is the use in ferroelectric liquid crystalmixtures, which are operated in inverse mode.

The invention additionally provides liquid-crystal mixtures, preferablyferroelectric and nematic mixtures, especially preferred ferroelectricand antiferroelectric, especially ferroelectric mixtures, comprising oneor more compounds of the formula (I).

The smectic and nematic liquid crystal mixtures are preferableapplicable for electrooptic displays, in the case of nematic mixturesespecially for active matrix displays and in-plane-switching displays(IPS-LCD), in the case of smectic liquid crystal mixtures for ECBdisplays, for electroclinic displays and chirally inclined smectic(ferroelectric or antiferroelectric) displays.

The novel liquid-crystal mixtures generally contain from 2 to 35,preferably from 2 to 25 and, with particular preference, from 2 to 20components.

They generally contain from 0.01 to 80% by weight, preferably from 0.1to 60% by weight, particularly preferably from 0.1 to 30% by weight, ofone or more, preferably from 1 to 10, particularly preferably from 1 to5, very particularly preferably from 1 to 3, of the novel compounds ofthe formula (I).

Further components of liquid-crystal mixtures comprising novel compoundsof the formula (I) are preferably selected from known compounds havingsmectic and/or nematic and/or cholesteric phases. These include, forexample:

derivatives of phenylpyrimidine as described, for example, in WO86/06401 and U.S. Pat. No. 4 874 542,

meta-substituted aromatic compounds having a six-membered ring, asdescribed, for example, in EP-A 0 578 054,

silicon compounds as described, for example, in EP-A 0 355 008,

mesogenic compounds having only one side chain, as described, forexample in EP-A 0 541 081,

hydroquinone derivatives as described, for example, in EP-A 0 603 786,

phenylbenzoates as described, for example, in P. Keller, Ferroelectrics1984, 58, 3 and J. W. Goodby et al., Liquid Crystals and Ordered Fluids,Vol. 4, New York, 1984, and

thiadiazoles as described, for example, in EP-A 0 309 514.

Examples of suitable chiral, nonracemic dopes are:

optically active phenylbenzoates as described, for example, in P.Keller, Ferroelectrics 1984, 58, 3 and J. W. Goodby et al., LiquidCrystals and Ordered Fluids, Vol. 4, New York, 1984,

optically active oxirane ethers as described, for example, in EP-A 0 263437 and WO-A 93/13093,

optically active oxirane esters as described, for example, in EP-A 0 292954,

optically active dioxolane ethers as described, for example, in EP-A 0351 746,

optically active dioxolane esters as described, for example, in EP-A 0361 272,

optically active tetrahydrofuran-2-carboxylic esters as described, forexample, in EP-A 0 355 561, and

optically active 2-fluoroalkyl ethers as described, for example, in EP-A0 237 007 and U.S. Pat. No. 5,051,506.

Suitable additional components of mixtures are set out in particular inthe international patent application PCT/EP 96/03154, which is expresslyincorporated herein by reference.

Preferred additional components of FLC mixtures used in the inverse modeare:

phenanthrene derivatives of the formula (III), ##STR9## fluoropyridinesof the formula (IV), ##STR10## difluorophenylene derivatives of theformula (V), ##STR11## meta-substituted aromatic compounds of theformula (VI), ##STR12## 4-cyanocyclohexyls of the formula (VII),##STR13## where the symbols and indices have the following meanings: X¹and X² are identical or different and independently of one another areCH, CF or N,

Y is F, CF₃ oder R

R and R¹ are identical or different and independently of one another areas defined for R¹ and R² in formula (I),

A and M are identical or different and independently of one another aredefined as in formula (I), and

a,b,c,d are identical or different and independently of one another are0 or 1, with the proviso that the compounds may contain no more thanfour ring systems and must, with the exception of the formula (III),contain at least two ring systems.

The mixtures can in turn be employed in electrooptical or fully opticalelements, for example display elements, switching elements, lightmodulators, elements for image processing and/or signal processing or,generally, in the area of nonlinear optics.

In addition the mixtures are suitable for field treatment, i.e. foroperation in the quasi-bookshelf geometry (QBG) (see, for example, H.Rieger et al., SID 91 Digest (Anaheim) 1991, 396).

The novel ferroelectric liquid-crystal mixtures are particularlysuitable for operation in the so-called inverse mode or tV.sub.(min)mode (see for example: J. C. Jones, M. J. Towler, J. R. Hughes, Displays1993, 14, No. 2, 86-93; M. Koden, Ferroelectrics 1996, 179, 121-129).

Liquid-crystalline mixtures comprising compounds of the formula (I) areparticularly suitable for use in electrooptical switching and displaydevices (displays). These displays are usually constructed in such a waythat a liquid-crystal layer is enclosed on both sides by layers whichare usually, in this sequence starting from the LC layer, at least onealignment layer, electrodes and a limiting sheet (for example of glass).In addition, they comprise spacers, adhesive frames, polarizers and--forcolor displays--thin color-filter layers. Other possible components areantireflection, passivation, compensation and barrier layers andelectric nonlinear elements, such as thin-film transistors (TFTs) andmetal-insulator-metal (MIM) elements. The structure of liquid-crystaldisplays has already been described in detail in relevant monographs(see, for example, E. Kaneko, "Liquid Crystal TV Displays: Principlesand Applications of Liquid Crystal Displays", KTK Scientific Publishers,1987).

The invention therefore additionally provides a switching and/or displaydevice, preferably a smectic or nematic, especially a ferroelectricdevice, comprising a liquid-crystal mixture which comprises one or morecompounds of the formula (I).

In devices, which contain nematic liquid crystal mixtures, active matrixdisplays and in-plane-switching displays (IPS-LCD) are preferred.

In devices, which contain smectic liquid crystal mixtures, ECB displays(electrically controlled birefringence), electroclinic displays andchirally inclined smectic (ferroelectric or antiferroelectric) displaysare preferred.

Such devices are e.g. applicable for computer displays or in chip-cards.

The novel switching and/or display device is preferably operated innormal mode or inverse mode.

Ferroelectric switching and/or display devices operated by multiplexaddressing can inter alia be operated in two different ways, the normalmode or the inverse mode (or tV.sub.(min) mode). The difference betweenthe two is in the addressing scheme and in the various requirementsregarding the dielectric tensor of the FLC material, i.e. the FLCmixture. An overview is given, for example, by J. C. Jones et al. inDisplays 1993, 14, No. 2, 86-93, referred to below as Jones, and M.Koden in Ferroelectrics 1996, 179, 121-129, and the literature set outtherein.

The response characteristics of an FLC device can be generallyrepresented in a diagram in which the driver voltage (V) is plottedhorizontally and the width of the driving pulse (t, time) is plottedvertically (see for example Jones, FIGS. 4, 8, 10 and 11).

A response curve is determined experimentally and divides the V,t areainto a switching region and a nonswitching region. Normally, the pulsewidth becomes shorter when the voltage is increased. It is this behaviorwhich characterizes the mode known as normal (see for example Jones,FIG. 4).

With appropriate materials, however, the Vt curve has a minimum (at thevoltage V.sub.(min)), as can be seen in Jones, for example, in FIGS. 8,10 and 11. This minimum comes about as the result of the superimpositionof dielectric and ferroelectric twisting. FLC devices are operated inthe inverse mode if the sum of the row driver voltage and column drivervoltage in the operating temperature range is higher than the minimum onthe Vt curve, i.e. V.sub.(row) +V.sub.(column) >V.sub.(min).

In the present application are cited various documents, e.g. toillustrate the technological surrounding of the invention. All thesedocuments are specially related to this invention, they are part of theapplication by reference.

Also related to the invention is the content of german application no.196 52 252.8, the priority thereof is claimed in this application, andthe abstract of this application; all are part of the application byreference:

The invention is illustrated in more detail by means of the followingexamples, although this is not intended to represent a limitation.

EXAMPLE 1

1,3-Difluoro-2-(4-hexyloxyphenyl)-6-octylnaphthalene ##STR14##

From 1,3-difluoro-6-octyinaphthalene-2-boronic acid and4-hexyloxybromobenzene by Suzuki coupling.

EXAMPLE 2

1,3-Difluoro-6-hexyl-2-(4-octylphenyl)naphthalene ##STR15##

From 1,3-difluoro-6-hexylnaphthalene-2 -boronic acid and4-octylbromobenzene by Suzuki coupling.

EXAMPLE 3

5-(1,3-Difluoro-6-octylnaphthalen-2-yl)-2-hexyloxypyridine ##STR16##

From 1,3-difluoro-6-octyinaphthalene-2- boronic acid and5-bromo-2-hexyloxypyridine by Suzuki coupling.

EXAMPLE 4

1,3-Difluoro-2-(2-fluorooctyloxy)-6-(4-hexylphenyl)naphthalene ##STR17##

From 1,3-difluoro-6-(4-hexylphenyl)naphth-2-ol and2-(S)-fluorooctan-1-ol by Mitsunobu reaction.

EXAMPLE 5

2-Butyl-3-[6-(2,3-difluoro-4-hexylphenyl)-1,3-difluoronaphth-2-yloxymethyl]-oxirane ##STR18##

From 6-(2,3-difluoro-4-hexylphenyl)-1,3-difluoronaphth-2-ol and(3-butyloxiranyl)methanol by Mitsunobu reaction.

EXAMPLE 6

2-{6-[4-(Butyidimethylsilanyl)butyl]-1,3-difluoronaphth-2-yl}-5-octyloxypyrimidine##STR19##

From2-{6-[4-(butyldimethylsilanyl)butyl]-1,3-difluoronaphth-2-yl}-pyrimidin-5-oland octylbromide by Williamson ether synthesis.

EXAMPLE 7

1,3-Difluoro-6-(4-hexyloxyphenyl)naphth-2-yl octanoate ##STR20##

From 1,3-difluoro-6-(4-hexyloxyphenyl)naphth-2-ol and octanoic acid byesterification with dicyclohexylcarbodiimide.

EXAMPLE 8

6-[5,7-Difluoro-6-(8-methyldecyloxy)naphth-2-yl]-2-fluoro-3-hexylpyridine##STR21##

From 1,3-difluoro-6-(6-fluoro-5-hexylpyridin-2-yl)naphth-2-ol and8-methyidecan-1-ol by Mitsunobu reaction.

EXAMPLE 9

1,3-Difluoro-6-hexylnaphth-2-yl 4-pentylcyclohexanecarboxylate ##STR22##

From 4-trans-pentylcyclohexancarboxylic acid and1,3-difluoro-6-hexylnaphth-2-ol.

What is claimed is:
 1. A 1,3-difluoronaphthalene derivative of theformula (I)

    R.sup.1 (--A.sup.1 --M.sup.1).sub.a (--A.sup.2 --M.sup.2).sub.b --B(--M.sup.3 --A.sup.3).sub.c (--M.sup.4 --A.sup.4).sub.d --R.sup.2 (I)

in which the symbols and indices have the following meanings: group B is##STR23## R¹ and R² are identical or different and are a) hydrogen, --F,--Cl, --CF₃ --OCF₃ or --CN, or b) a straight-chain or branched alkylradical (with or without an asymmetric carbon atom) having 1 to 20carbon atoms, whereb₁) one or more nonadjacent and nonterminal CH₂groups can be replaced by --O--, --CO--O, --O--CO-- or --Si(CH₃)₂ --,and/or b₂) one or more CH₂ groups can be replaced by --CH═CH--,--C═--C--, 1,4-phenylene, 1,4-cyclohexylene and/or b₃) one or more Hatoms can be replaced by F and/or b₄) the terminal CH₃ group can bereplaced by one of the following chiral groups (optically active orracemic): ##STR24## with the proviso that at most one of the radicalsR¹, R₂ can be hydrogen, --F, --Cl, --CF₃, --OCF₃ or --CN; R³ is astraight-chain or branched alkyl radical (with or without an asymmetriccarbon atom) having 1 to 16 carbon atoms, where1) one or morenonadjacent and nonterminal CH₂ groups can be replaced by --O--, and/or2) one or two CH₂ groups can be replaced by --CH═CH--; M¹, M², M³, andM⁴ are identical or different and are --CO--O--, O--CO--, --CH₂ --O--,--O--CH₂ -- --C.tbd.C--, or a single bond; A¹, A², A³, and A⁴ areidentical or different and are 1,4-phenylene in which one or more Hatoms can be replaced by F, pyridine-2,5-diyl in which one or more Hatoms can be replaced by F, pyrimidine-2,5-diyl in which one H atom canbe replaced by F, 1,4-cyclohexylene in which one or two H atoms can bereplaced by CN and/or CH₃ and/or F, 1 ,3-dioxane-2,5-diyl,naphthalene-2,6-diyl in which one or two H atoms can be replaced by F,Cl and/or CN, or 1-(C₁ -C₄)alkyl-1-silacyclohexylene-1,4-diyl; a, b, c,and d are 0 or 1; with the proviso that the compound of the formula (I)contains no more than four ring systems having five or more members,with the exception of compounds having not more than 3 five- or moremember ring systems, and R¹ C₁₋₂₀ -alkyl in which non terminal nonneighboring CH₂ -groups can be replaced by --O--, wherein the CH₂ -groupdirectly bound to A¹, A² or B is replaced by --CH═CH--, and M¹ =M² =M³=M⁴ =single bond, or R² C₁₋₂₀ -alkyl in which non terminal nonneighboring CH₂ -groups can be replaced by --O--, wherein the CH₂ -groupdirectly bound to A⁴, A³ or B is replaced by --CH═CH--, and M¹ =M² =M³=M⁴ =single bond.
 2. A 1,3-difluoronaphthalene derivative as claimed inclaim 1, wherein the symbols and indices in the formula (I) have thefollowing meanings:R¹ and R² are identical or different and area)hydrogen, or b) a straight-chain or branched alkyl radical (with orwithout an asymmetric carbon atom) having 1 to 18 carbon atoms, whereb₁)one or more nonadjacent and nonterminal CH₂ groups can be replaced by--O--, --CO--O--, --O--CO--, or --Si(CH₃)₂ --, and/or b₂) one CH₂ groupcan be replaced by 1,4-phenylene, or trans-1,4-cyclohexylene and/or b₃)one or more H atoms can be replaced by F and/or b₄) the terminal CH₃group can be replaced by one of the following chiral groups (opticallyactive or racemic): ##STR25## with the proviso that at most one of thetwo radicals R¹ and R² can be hydrogen; R³ is a straight-chain orbranched alkyl radical (with or without an asymmetric carbon atom)having 1 to 14 carbon atoms, where one or more nonadjacent andnonterminal CH₂ groups can be replaced by --O--; M¹, M², M³, and M⁴ areidentical or different and are --CO--O--, O--CO--, --CH₂ --O--, --O--CH₂--, or a single bond; A¹, A², A³, and A⁴ are identical or different andare 1,4-phenylene in which one or two H atoms can be replaced by F,pyridine-2,5-diyl in which one H atom can be replaced by F,pyrimidine-2,5-diyl in which one H atom can be replaced by F, ortrans-1,4-cyclohexylene; a, b, c, and d are 0 or 1; with the provisothat the compound of the formula (I) contains no more than four ringsystems having five or more members.
 3. A 1,3-difluoronaphthalenederivative as claimed in claim 1, wherein the symbols and indices in theformula (I) have the following meanings:R¹ and R² are identical ordifferent and area) hydrogen, or b) a straight-chain or branched alkylradical (with or without an asymmetric carbon atom) having 1 to 16carbon atoms, where one or more nonadjacent and nonterminal CH₂ groupscan be replaced by --O--;with the proviso that only one of the radicalsR¹ and R² can be hydrogen; M¹, M², M³, and M⁴ are identical or differentand are --CH₂ --O--, --O--CH₂ -- or a single bond; A¹, A², A³, and A⁴are identical or different and are 1,4-phenylene in which one or two Hatoms can be replaced by F, pyridine-2,5-diyl in which one H atom can bereplaced by F, pyrimidine-2,5-diyl, or trans-1,4-cyclohexylene; a, b, c,and d are 0 or 1; with the proviso that the compound of the formula (I)contains no more than four ring systems having five or more members. 4.A 1,3-difluoronaphthalene derivative as claimed in claim 1, which isselected from the group consisting of (Ia)-(Ii): ##STR26## in which R¹and R² have the meanings stated in formula (I) in claim 1, with theproviso that at most one of the radicals R¹ and R² can be hydrogen.
 5. Aliquid-crystal mixture comprising one or more 1,3-difluoro-naphthalenederivatives as claimed in claim
 1. 6. A liquid-crystal mixture asclaimed in claim 5, which is ferroelectric.
 7. A liquid-crystal mixtureas claimed in claim 5, which contains from 0.01 to 80% by weight of oneor more 1,3-difluoronaphthalene derivatives of the formula (I).
 8. Aferroelectric switching and/or display device comprising a ferroelectricliquid-crystal mixture as claimed in claim
 6. 9. A ferroelectricswitching and/or display device as claimed in claim 8, which is operatedin the _(T) V_(min) mode.